The helical distortion of a compact tokamak plasma with currents in modularFurth-Hartman type coils is investigated with respect to the impact on themagnetic field structure and to the ideal magnetohydrodynamic stability. Asequence of configurations, in which the current in the helical coils isincreased up to the level of that of the main toroidal coils, is generated.The magnetic field structure acquires a ripple along the field lines when thecurrents in both sets of coils becomes comparable. The magnitude of thisripple depends on the modulation of the Furth-Hartman coils on their windingsurface. The local ideal ballooning stability properties deteriorate with theincreasing helical deformation of the plasma shape because the normal magneticfield line curvature becomes larger in regions where the helical contributionenhances the toroidicity component while the local magnetic shear vanishes.The global external kink stability properties, in contrast, improvebecause the normal curvature on average does not change while the localmagnetic shear acquires a noticeable helical modulation.
Toroidal effects on a helical pinch configuration with pitch reversal